Printed circuit board connectors for coupling daughter cards to backplanes are well known in the art. However, the electrical properties of these connectors have historically limited high-speed designs. The simultaneous requirements for robust connectivity, serviceability, and excellent electrical properties have resulted in a series of design trade-offs that delivered less than optimal signal performance, particularly in the 90-degree configurations often used to connect a backplane to a daughter card.
Prior art demonstrates that many and various constructions have been utilized in order to achieve a 90-degree signal connection with good electrical properties. These constructions fall into several broad categories:
1. Constructions using stamped or formed metal pins, leads, or contacts to propagate a signal from one board interface to another board interface. These are further broken down into various types: PA1 A. Pin-field connector. These connectors utilize an array of signal pins to provide the signal path, and count on a certain number of those pins being connected to ground in order to enable a robust path for return currents. In high speed connections, these often have problems due to the large loop areas between signals and grounds, and the fact that the dielectric material varies from plastic to air at many intervals along the connected path. In addition, the inherent inductance of the metal pin per unit length is generally poorly matched with the inherent capacitance of the dielectric per unit length. As a result, the interconnects tend to be inductively dominated, and the resulting impedance at high frequencies is different than the desired impedance. This results in degraded signal quality across the connector. Signal-to-signal crosstalk coupling is typically high in this type of interconnect, further degrading signal quality. PA1 B. Stripline and Microstrip connectors using metal leads. This family of connectors are built around a planar structure, which is generally composed of a stamped or formed metal plate. The metal signal conductors are positioned in a deterministic relationship relative to the metal plate in a manner such that a pseudo stripline or pseudo microstrip structure is formed. The electrical characteristics of this type of connector are much better than those of pin field connectors, but still fall short of the electrical properties of the printed circuit board (PCB). Causes of signal degradation in this type of connector include mixed air/plastic dielectric, point discontinuities associated with contact features, and the amount of capacitance associated with large through-hole vias. PA1 C. Pseudo-coax and pseudo-twinax constructions. These connectors emulate the physical construction of a shielded conductor cable by surrounding a stamped or formed metal lead with a complete or partial shield. The shield can be constructed of metal, or alternatively be a molded plastic assembly plated, painted, or coated with a metal conductive shield. As with the previous constructions, the metal lead is surrounded with a combination of plastic and air dielectric. These connectors offer very good electrical performance when compared to pin fields, but do not provide the level of performance found in PCBs. Causes of signal degradation in this type of connector include mixed air/plastic dielectric, point discontinuities associated with contact features, and the amount of capacitance associated with large through-hole vias. PA1 2. Constructions using flexible circuits. These connectors address signal quality requirements by producing a printed circuit construction, complete with ground planes, within the confines of the 90-degree contact. This is achieved by manufacturing flat sections of circuit material, and then bending them to 90 degrees. Various types of contacts have been used, including gold bumps, rubber elastomers pressing against gold pads, and heat-activated springs. While flexible circuit performance can help address some of the issues associated with stamped or formed metal leads, they have their own problems: PA1 A. A stripline construction requires three conductor layers, while the standard materials and processes produce two layers. As a result, microstrip constructions are often used, that have higher crosstalk and more EMI radiation than stripline. Alternatively, stripline structures are produced in a layer stackup that is more difficult to produce and more difficult to bend than non-stripline structures because of the skin structure formed by the two outer copper ground layers. PA1 B. Because of the difficulty in manufacturing multi-layer, controlled-impedance flexes with more than one stripline layer, yields are very low for interconnects offering high density. PA1 C. In order to achieve density with a low layer count, many flex interconnects are produced with extremely fine lines and close spacing between lines. Fine lines add attenuation to the signal path, and close spacing increases crosstalk. PA1 D. The flexible nature of the contact materials requires that the connector have specific and often expensive alignment hardware to ensure connectivity between flex pads and board features. PA1 3. Constructions using coax or twinax cable. Used under the most stringent performance conditions, these connectors are constructed of sections of cable materials attached to small PCBs or plastic contact carriers. (For example see Schumacher, U.S. Pat. No. 5,823,795.) While signal quality of these connectors is quite good, manufacturing costs are usually prohibitive for all but the most expensive systems.
There is a need in the art for a high-performance, easily manufacturable, connector with electrical properties matched to the system. This connector must be mechanically robust, inexpensive, and available in a wide range of sizes.